This invention relates to a method for producing multi-figure luminescent display tubes and, in particular, to a method for producing such tubes which can be operated by lower electric power and voltage.
As is well known, a multi-figure luminescent display tube comprises an insulator, and a plurality of groups of segment electrodes. Each group of segment electrodes displays various graphic patterns such as numerals, alphabetic letters or other figures. Each segment electrode is provided with a phosphor layer on the surface thereof. The plurality of groups of segment electrodes are arranged in a plane and are mounted on the insulator.
Filaments are supported on the insulator and extend over, and are spaced from, the plurality of groups of segment electrodes. Between the filaments and the segment electrodes, a plurality of grid electrodes are so disposed that each grid electrode may be over a particular group of segment electrodes. The grid electrodes are, also, supported on the insulator.
The segment electrodes, grid electrodes and filaments are contained in an airtight evacuated envelope. At least one portion of the envelope is transparent so that segment electrodes may be viewed from outside the envelope.
Leads for the filaments, grid electrodes and segment electrodes extend through the insulator to the outside of the envelope.
This display tube is produced by following steps. First, by insulator is prepared and provided with segment electrodes with phosphor layers and leads. Grid electrodes and filament supports are mounted and fixed on the insulator by frit glass. Filaments and getter rods are the fixed to, and supported on, filament supports.
Thereafter, the insulator is fixed to an envelope with a gas exhaust pipe by a frit glass seal so that the segment electrodes, grid electrodes, filaments and getter rods are contained in the envelope. Air in the envelope is exhausted by using the exhaust pipe under the glass baking condition for a predetermined duration, and the exhaust pipe is sealed at a predetermined length.
After the exhaust pipe is sealed off, the getter rods are fashed to evacuate the envelope sufficient to operate the display. Thus the display tube is obtained.
In the step of fixing grid electrodes and filament supports onto the insulator, the frit glass including binder is applied to the grid electrodes, filament supports and the insulator and, then, the frit glass is baked to strongly fix the parts onto the insulator.
The baking step is carried out in oxygen or air to remove the binder by combustion thereof, so that the phosphor layers may be protected from contamination by the binder.
We have found that the baking process in oxygen or air prevents the fabrication of a luminescent display tube having a sufficient brightness. Particularly, phosphor P15 or P24 (designation by the U.S. Radio Manufacturer's Association) used for luminescent display tubes consists of a mixture of ZnO and Zn, and is active. The ratio of ZnO to Zn and the crystal form thereof are varied by heating the phosphor in an oxidizing environment so that the resultant brightness may be varied.
Moreover, the frit glass presents a yellow-white appearance after baking in the oxidizing environment. Accordingly, the frit glass disturbs luminescent display by reflection.